Such a pump is known as an eccentric screw pump and is based on the idea by Rene Moineau described in the German patent DE 602,017. This patent describes a device comprising an outer part and an inner part each provided with screwthreads, the outer part having exactly one screwthread more than the inner part. The ratio of the pitch on the outer part to the pitch on the inner part in every cross-section corresponds to the ratio of the number of screwthreads on the respective parts. The inner and outer parts thus form closed pumping chambers that move axially from the intake end to the output end during relative rotation of the inner part in the outer part. If this device is to be used as a pump, according to the comments in DE 602,017 the device must always have at least one closed pumping chamber between the intake end and output end. This means that the screwthreads of the outer part must have an entire turn.
Various designs of eccentric screw pumps are known. For example, the inner part may be driven and act as the rotor, while the outer part remains stationary and is the stator. However, designs comprising an outer rotor and an inner stator are also possible. In both variants, designs are possible in which either the inner part moves radially eccentrically relative to the outer part, for example as in DE 602,017, or the outer part moves radially eccentrically relative to the inner part. The latter variant, which also comprises a rotatably driven inner part, is described in U.S. Pat. Nos. 2,612,845 and 2,691,347. In the eccentric screw pumps disclosed there, the stationary outer part (the pump stator) is made of an elastic material and is deformed so as to move on a circular path around the central pump rotor axis and thereby compensate for the eccentricity between the inner rotating part (pump rotor) and the outer part. The outer part is therefore also referred to as a vibrating stator or wobble stator. In this way, the joint in the drive train necessary in the otherwise standard eccentric screw pumps comprising pipe stators may be dispensed with, so that eccentric screw pumps comprising wobble stators can be produced considerably shorter and less expensively.
Eccentric screw pumps belong to the group of rotary positive displacement equipment. During operation, they continuously pump medium with the pumping chambers, which is to say by means of the enclosed pumping chambers between the inner part and the outer part, to the output end. If an enclosed volume is present on the output end, for example due to a closed valve, in particular a flow restriction, pressure builds continuously in the enclosed volume. Without suitable measures, the pump will continue to deliver medium to be displaced through the moving, closed pumping chambers from the intake end to the output end, and the pressure rises drastically at the output. In the waste water technology field, in which the described pumps are preferably used, output-end throttling can be caused by a closed gate valve, for example, or else by the accumulation of fragments of the waste water in the pipe. The pressure buildup at the output-end can result in destruction of the pump or of the system connected to the pump.
So as to protect the pressurized system, monitoring devices such as pressure sensors must be provided on the one hand, and protective measures such as deactivation of the pump or pressure relief devices must be provided on the other hand. The latter may be safety valves, bursting diaphragms or bypass lines that can be opened, for example.
Safety valves are associated with the risk of becoming clogged with fragmented components of the waste water, which can accumulate in the flow-conducting components of the valve. Bursting diaphragms relieve a pressurized system by being destroyed when the bursting pressure is applied and by exposing an opening so that the pressurized medium to be displaced can be discharged. Replacement of the destroyed bursting diaphragm is required to put the system back in operation. In addition, the service life is dependent on the ratio of the actual operating pressure to the bursting pressure, so that the bursting diaphragm may become pre-damaged, for example as a result of brief pressure surges, and then bursts at a lower pressure. Another disadvantage of bursting diaphragms is the relatively high price. A closed valve or a clogged location downstream of the pump can be circumvented by using a bypass line. However, this necessitates increased piping complexity, requires installation volume, and thus results in increased installation costs.